The Grim Adventures of Billy and Mandy (2001)

S1E14 A Grim Surprise

Billy enlists Grim's assistance to organize a surprise birthday bash for Mandy. Although hesitant, Grim agrees to help Billy whip up a cake while ensuring Mandy remains in the dark. When the big moment arrives, Mandy is taken aback but then clarifies that her birthday isn't for another five months. It turns out Billy was aware of this all along.